During this period, two seasonal thermal energy storage (sTES) technologies (Reno-sTES and Giga-CTES) were advanced using a unified methodology adapted to site-specific and technological conditions. Tailored experimental, digital, economic, and environmental approaches were developed, and preparation began at both sites. Monitoring was installed across storage, energy systems, and ambient domains. Integrated planning and execution enabled coordinated data acquisition, while early activities focused on extensive characterisation measures. These included technological, geological, economic, and environmental methods. Thus, comparability supporting assessments of cost, performance, and replicability can be ensured as planned.
At the Reno-sTES site, existing basins are being converted into cost-efficient water-gravel thermal storage, with completion targeted for autumn 2025. Existing materials are reused in line with circular economy principles. Refined monitoring with thermal and moisture sensors is included, and the connection to the energy system is prepared. Real-time data will feed into INTERSTORES’ digital twin framework to support replicable, scalable evaluation. WGTES fillings and insulation materials were assessed for moisture resistance and long-term performance.
At the Giga-CTES site, construction of the world’s largest cavern thermal energy storage system is set to begin in late 2025, with commissioning expected in 2028. In line with a refined design based on techno-economic optimisation, geological and system investigations were conducted. Monitoring equipment was installed around the site, with further installations planned inside the storage. These will support the digital twin during the heat-up phase. Components were studied for durability under extreme conditions using autoclave and electrochemical methods.
A modular framework was established to integrate ground, storage, and energy system models using simplified approaches validated with detailed datasets. These models support optimisation, sensitivity analysis, predictive control, and fault detection. Basin compound and heat exchanger simulations were conducted for Reno-sTES, while a flexible variant was prepared for the digital twin. For the Giga-CTES, simulations refined diffuser designs, and a multiphysics model is being established in alignment with the construction timeline. Energy system models were developed to ensure efficient sTES integration. Regional demand data were used for the Giga-CTES. For the Reno-sTES, potentials were studied through a local model. Digital control and optimisation tools are under development. Subsurface models for both sites capture key processes, including geological features, thermal-hydraulic-mechanical behaviour, and porous media interactions. These are being refined with field data to inform planning and operation.
Environmental performance was addressed through development of a flexible Life Cycle Assessment framework. Site-specific inventories of material, energy, and resource use employed early design data and are updated during construction. A recycling strategy was initiated to align with circular economy objectives, supporting a comprehensive environmental assessment across sTES variants.
A techno-economic evaluation was launched to assess all relevant life cycle costs. Supporting documentation and the planned user guide are in early preparation. Stakeholder mapping, spatial analyses, and geological assessments are in progress to identify potential replication sites. On this basis, market readiness and opportunities for broader sTES deployment across Europe will be explored.
